java开发中几种常见的线程池

线程池

java.util.concurrent:Class Executors

常用线程池

几种常用的的生成线程池的方法：

• newCachedThreadPool
• newFixedThreadPool
• newScheduledThreadPool
• newSingleThreadExecutor
• newSingleThreadScheduledExecutor

例子：newFixedThreadPool

ExecutorService threadPool = Executors.newFixedThreadPool(3);for(int i=0;i<10;i++){    threadPool.execute(new Runnable() {        @Override        public void run() {            System.out.println(Thread.currentThread().getName());        }    });}

单线程newSingleThreadExecutor可用于重启

用线程池启动定时器

例子：类似Timer的定时执行

Executors.newScheduledThreadPool(3).scheduleAtFixedRate(            new Runnable() {                @Override                public void run() {                    System.out.println("ScheduledThreadPool "+Thread.currentThread().getName());                }            },3,1, TimeUnit.SECONDS    );

Callable&Future

ExecutorService在Executor的基础上增加了一些方法，其中有两个核心的方法：

• Future<?> submit(Runnable task)
• <T> Future<T> submit(Callable<T> task)

这两个方法都是向线程池中提交任务，它们的区别在于Runnable在执行完毕后没有结果，Callable执行完毕后有一个结果。这在多个线程中传递状态和结果是非常有用的。另外他们的相同点在于都返回一个Future对象。Future对象可以阻塞线程直到运行完毕（获取结果，如果有的话），也可以取消任务执行，当然也能够检测任务是否被取消或者是否执行完毕。

Lock&Condition

Lock

Lock功能类似传统多线程技术里的synchronized，实现线程互斥，但更加面向对象。将需要互斥的代码片段放到lock.lock();和lock.unlock();之间。

例子

class A{    private Lock lock = new ReentrantLock();    public void function(){        lock.lock();        try{            //功能代码        }finally{            lock.unlock();        }    }}

• 读写锁

java.util.concurrent.locks:Class ReentrantReadWriteLock

javaDoc文档读写锁例子,缓存:

class CachedData {   Object data;   volatile boolean cacheValid;   final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();   void processCachedData() {     rwl.readLock().lock();     if (!cacheValid) {       // Must release read lock before acquiring write lock       rwl.readLock().unlock();       rwl.writeLock().lock();       try {         // Recheck state because another thread might have         // acquired write lock and changed state before we did.         if (!cacheValid) {           data = ...           cacheValid = true;         }         // Downgrade by acquiring read lock before releasing write lock         rwl.readLock().lock();       } finally {         rwl.writeLock().unlock(); // Unlock write, still hold read       }     }     try {       use(data);     } finally {       rwl.readLock().unlock();     }   } }

重点注意在释放写锁前加读锁那部分代码，注释为// Downgrade by acquiring read lock before releasing write lock。自己挂了写锁，再挂读锁是可以的，这面涉及的技巧以后再研究。

Condition

Condition类似于传统多线程技术中的Object.wait和Object.notify,实现线程间同步。

javaDoc文档例子，可阻塞队列

class BoundedBuffer例子

class BoundedBuffer {   final Lock lock = new ReentrantLock();   final Condition notFull  = lock.newCondition();    final Condition notEmpty = lock.newCondition();    final Object[] items = new Object[100];   int putptr, takeptr, count;   public void put(Object x) throws InterruptedException {     lock.lock();     try {       while (count == items.length)         notFull.await();       items[putptr] = x;       if (++putptr == items.length) putptr = 0;       ++count;       notEmpty.signal();     } finally {       lock.unlock();     }   }   public Object take() throws InterruptedException {     lock.lock();     try {       while (count == 0)         notEmpty.await();       Object x = items[takeptr];       if (++takeptr == items.length) takeptr = 0;       --count;       notFull.signal();       return x;     } finally {       lock.unlock();     }   } }

使用了两个condition

同步工具

• Semaphore

类似占坑

• CyclicBarrier

阶段性使进度一致

• CountDownLatch

一人通知多人/多人通知一人

• Exchanger

线程间数据交换，都到达则自然交换